Re: NFS BUG_ON in nfs_do_writepage

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Tue, 2009-04-28 at 06:27 +0200, Nick Piggin wrote:
> On Sun, Apr 26, 2009 at 01:55:22PM -0400, Trond Myklebust wrote:
> > On Sun, 2009-04-26 at 17:13 +0200, Nick Piggin wrote:
> > > This doesn't seem to fix the race, though... on kernels with the
> > > race still there, it will just open a window where you can have
> > > a dirty pte but the page not written out.
> > > 
> > > I don't understand.
> > 
> > I'm just pointing out that the NFS client already calls
> > __set_page_dirty_nobuffers() while holding the page lock inside the
> > nfs_vm_page_mkwrite() call, so having the VM do it too in the call to
> > set_page_dirty_balance() is actually redundant. IOW: as far as the NFS
> > code is concerned, we can get rid of the ->set_page_dirty() callback in
> > that situation.
> > 
> > I couldn't find any other places in the VM code where we can have a
> > dirty pte without also having called page_mkwrite() (and hence
> > __set_page_dirty_nobuffers). As I said, adding a WARN_ON(!PageDirty())
> > in ->set_page_dirty() didn't ever trigger any cases where the
> > set_page_dirty() was actually setting the dirty bit (except in the case
> > where we race with page writeout in do_wp_page() and __do_fault()).
> > 
> > That's why I believe disabling ->set_page_dirty() is safe here, and will
> > in fact suffice to fix the page writeout race.
> 
> Ah, no I don't think so because it opens another race where the
> pte is dity but the page is marked clean.

So how can that happen?

AFAICS, when the pte is dirtied, we should get a page fault, which
causes the page itself to be marked dirty by the nfs_vm_page_mkwrite()
callback.
When the page gets written out, the VM calls clear_page_dirty_for_io()
which also causes the pte to be cleaned.

At what point can you therefore have a situation where the pte is dirty
without the page being marked as dirty too?